You can use the built-in flash, hot-shoe flash, external flash, or radio triggers. The internal flash controls remain disabled until you pop-up the internal flash on the camera. The internal flash controls can only control the internal built-in flash. Repeating flash is not supported. To show the internal flash controls, go to the View menu and select ‘Flash’.
Many older bodies support TTL, Manual and Commander mode. However, newer bodies support only TTL mode, and some bodies won’t support any remote flash control at all (such as the D5200), even though it has a built-in flash. We’re not sure why Nikon has restricted this in their SDK.